The invention generally relates to limited rotation motors (sometimes referred to as galvanometer motors), and relates in particular to systems for controlling the movement of limited rotation motors.
Limited rotation motors move within a limited radial range of motion only. Certain limited rotation motor systems employ a torsion bar, one end of which is fixed and the other end of which moves with the limited rotation motor shaft. The motor acts against a force caused by the torsion bar that resists rotation. When the motor drives the shaft to one swing of its motion range and then turns off, the torsion bar applies torque to the shaft to return the shaft to its neutral position.
For example, FIG. 1 shows a limited rotation motor system 10 that includes a clamp hub 12, and a torsion bar 14 that is fixed to the clamp hub 12. The system also includes a torsion bar hub 16, and a magnetic rotor 18 (having a generally rectangular cross-sectional area) on a shaft that is supported by a rear bearing 20 and a front bearing 22. The front bearing is fixed to the housing frame, while the rear bearing is slidably attached to the housing frame. The clamp hub 12, torsion bar 14, and torsion hub 16 are spot welded together. The torsion hub 16 and rotor 18 are soldered together. The torsion bar stores potential energy when the rotor 18 is rotated, and releases the energy when the power to the motor (stator windings) is turned off.
The bearings are typically manufactured with a degree of manufacturing tolerance that provides a small amount of play between the balls and the bearing races or rings. This play may be reduced by a process called radial pre-loading that applies a force vector across the bearing, making the outer ring eccentric with respect to the inner ring; and this eccentricity is limited by the presence of a ball or balls trapped between the rings. For example, FIG. 2 shows a bearing having an inner ring 30, an outer ring 32, and a plurality of bearing balls 34 between the inner and outer rings. Radial pre-loading involves establishing a force vector as shown at A that brings the inner and outer rings closer together at a location in an effort to reduce unwanted play in the bearing system. Such radial pre-loading may be achieved, for example, by including a small asymmetric notch in the rotor 18 as shown at 24. This imbalance causes asymmetric radial magnetic forces to be applied to the rotor, which creates the force as indicated at A.
It has been found, however, that radial pre-loading does not fully remove play in certain bearing systems. It has further been found that the use of a torsion bar may provide excessive resistive torque forces near the range limit of certain limited rotation motors. Also, the very high axial stiffness of the torsion bar causes Brinelling of the front bearing under conditions of temperature extreme due to the difference in coefficient of thermal expansion between the torsion bar and the stator frame.
There is a need therefore, for a limited rotation motor control system for providing improved performance of a limited rotation motor. There is further a need for an improved limited rotation motor system that is efficient and economical to manufacture.